1. Field of the Invention
This invention relates to a transmitter-and-receiver device housing an antenna therein, and more particularly to an improved device applicable to a receiver of a battery-type keyless entry system mounted on a vehicle.
2. Discussion of the Related Art
There are conventionally employed a Yagi antenna or a helical antenna as a UHF beam antenna particularly in a band between 300 MHz and 400 MHz. Since the Yagi antenna is bulky and cannot be installed within a small transmitter-and-receiver device, the helical antenna is employed as an antenna internally installed in such a device.
FIG. 19 shows a schematic perspective view of a conventional transmitter-and-receiver device 201 housing a pair of helical antennas 202, and FIG. 20 shows a schematic front view of the device. An earthing conductor 204 is disposed below a lower wall of a print board 203 and a transmit-and-receive circuit section (not shown in drawings) is mounted on an upper wall of the board 203. The pair of helical antennas 202 are connected in parallel and disposed in an orthogonal relationship wherein helical axes thereof are in parallel with the print board 203. Each one end of the helical antennas 202 is connected with a feeder print pattern 205 connected with the transmit-and-receive circuit section.
The helical antenna 202 is of a dipole type to serve both as a loading coil function for increasing an electric length of an antenna (viz. for decreasing the antenna length) and an antenna function, but not satisfactory for miniaturizing and thinning the transmitter-and-receiver device 201. In order to miniaturize and thin the transmitter-and-receiver device 201 housing the helical antenna 202, it can be proposed to decrease a coil diameter D and a whole length L of the helical coil 202 and to lower a whole height H of the antenna.
Decreasing the coil diameter D and the whole length L of the helical antenna 202 decreases a volume (an effective volume of the antenna) for receiving and transmitting a radio wave, whereby unfavorably the receive sensitivity of the transmitter-and-receiver device 201 is lowered and the transmit power is decreased.
A radio wave is radiated from a slit space between the helical antenna 202 and the print board 203. As the whole height H of the helical antenna 202 is lowered, the gap "d" is decreased and a transmit-and-receive resistance (radiation resistance) is increased so that a radio wave is hard to be radiated or injected.
The helical antenna 202 has the disadvantages that an input impedance viewed from the feeder print pattern or portion 205 is too small to make an impedance matching and its loss is large because the electrical length of the antenna has to be designed to be 1/4 of a wavelength in use.
In FIG. 22 there is shown a conventional transmitter-and-receiver device 207 provided with an antenna easy for impedance matching. In the device 207, a rear end of an antenna body 209 disposed in parallel with a print board 208 is bent toward the board 208, and an earth end 210 formed on the rear end is fixed to the print board 208 to be electrically connected to an earthing conductor 211 of the board. A front end of the antenna body 209 is an open end 212. A feeder line 214 is extending from a middle of a portion of the antenna body 209 in parallel with the board 208 to connect the antenna 209 with a transmit-and-receive circuit 213. A tip end of the feeder line 214 is fixed to the print board 208 to be connected with the transmit-and-receive circuit 213 disposed on the board.
Thus, F-shaped antenna 215 is formed by the antenna body 209 and the feeder line 214, and provided with the earth end 210 at one end thereof across the feeder line 214 and the open end 212 at another end thereof. The input impedance can be easily adjusted by moving the feeder position on the antenna body 209 from the earth end 210 and the open end 212. The transmitter-and-receiver device 207 provided with the antenna 215 is not proper for miniaturizing the antenna and the device because the whole length of the antenna must be designed to be .lambda./4 of a desired frequency.